Our principal objective is the determination of the three-dimensional structures of immunoglobulins from patients with multiple myeloma and/or amyloidosis. We are currently investigating a serum IgG1 protein from the same patient (Mcg) who provided the urinary Bence-Jones dimer for which we previously solved the structure and constructed an atomic model. By selecting the IgG1 protein from Mcg, we can make extensive use of the known structure of Bence-Jones protein, which has the same amino acid sequence as the light chains in the parent IgG2 molecule. The Bence-Jones dimer is sufficiently similar to antigen-binding fragments (Fab) in size, shape, and the ability to bind hapten-like molecules that it can be considered a model for a primitive antibody. A continuing comparison of the binding properties of the Mcg Bence-Jones dimer and the IgG1 protein has provided important insights into the structural basis of antibody specificity. To extend these studies, we have obtained crystals of a human (Zie) IgG2 cryoglobulin and its F(ab')2 fragment from D. Parr and G. Connell at the University of Toronto. The close similarities in the crystallographic properties of the Zie proteins indicate that the Fc region does not contribute significantly to the diffraction pattern of the IgG2 immunoglobulin.